Voltage to Current Converter (V to I Converter)

Contents

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Key learnings:

Voltage to Current Converter Definition: A voltage to current converter is defined as an electronic circuit that converts an input voltage into a proportional output current.
Purpose of V to I Converters: These converters are essential in creating stable DC current signals for analog representation of physical quantities, ensuring less noise and constant current flow.
Ohm’s Law and Basic V to I Conversion: A resistor in a voltage source circuit can act as a simple V to I converter based on Ohm’s Law, though it may be imperfect.
Op-Amp Based Converters: Operational amplifiers like IC LM741 are used for more precise voltage to current conversion by maintaining the desired current through feedback mechanisms.
Types of V to I Converters: There are floating load converters (current series negative feedback amplifier) and ground load converters (Howland Current Converter), each with unique configurations and applications.

What is a Voltage to Current Converter (V to I Converter)?

A voltage to current converter (V to I converter) is defined as an electronic circuit that converts an input voltage into a proportional output current.

But why would we do this?

For instrumentation circuits, DC current is preferred for creating an analog representation of physical quantities like weight, pressure, and motion.

DC current signals remain constant throughout the circuit from the source to the load, and current-sensing instruments have the advantage of generating less noise.

So sometimes it is essential to create current which is corresponding or proportional to a definite voltage.

For this purpose Voltage to Current Converters (also known as V to I converters) are used. It can simply change the carrier of electrical data from voltage to current.

Simple Voltage to Current Converter

When we confer about the connection between voltage and current, it is obvious to mention the Ohm’s law.

We all know that when we supply a voltage as input to a circuit which comprises of a resistor, the proportional current will commence to flow through it.

So, it is clear that the resistor decides the current flow in a voltage source circuit or it performs as a simple voltage to current converter (i.e. a V to I converter) for a linear circuit.

The circuit diagram below shows a resistor acting as a simple voltage to current converter. In this diagram, voltage and current are represented by bars and loops, respectively.

But practically, the output current of this converter depends directly on the voltage drop across the connected load in addition to the input voltage. Since, V_R becomes . This is the reason why this circuit is said to be an imperfect one or bad or passive version.

Voltage to Current Converter Using Op-Amp

An op-amp is implemented to simply convert the voltage signal to corresponding current signal. The Op-amp used for this purpose is IC LM741.

This Op-amp is designed to hold the precise amount of current by applying the voltage which is essential to sustain that current through out the circuit. They are of two types that are explained in detail below.

Floating Load Voltage to Current Converter

As the name indicates, the load resistor is floating in this converter circuit. That is, the resistor R_L is not linked to ground.

The voltage, V_IN which is the input voltage is given to the non-inverting input terminal. The inverting input terminal is driven by the feedback voltage which is across the R_L resistor.

This feedback voltage is determined by the load current and is in series with the input difference voltage (VD). This circuit is also known as a current series negative feedback amplifier.

For the input loop, the voltage equation is

Since A is very large,
So,

Since, the input to the Op-amp,

From the above equation, it is clear that the load current depends on the input voltage and the input resistance.

That is, the load current, , which is the input voltage. The load current is controlled by the resistor, R. Here, the proportionality constant is 1/R.

So, this converter circuit is also known as Trans-Conductance Amplifier. Other name of this circuit is Voltage Controlled Current Source.

The type of load may be resistive, capacitive or non-linear load. The type of load has no role in the above equation.

When the load connected is capacitor then it will get charge or discharge at a steady rate. Due to this reason, the converter circuit is used for the production of saw tooth and triangular wave forms.

Ground Load Voltage to Current Converter

This V to I converter is also known as Howland Current Converter. Here, one end of the load is always grounded.

For the circuit analysis, we have to first determine the voltage, V_IN and then the relationship or the connection between the input voltage and load current can be achieved.

For that, we apply Kirchhoff’s current law at the node V₁

For a non-inverting amplifier, gain is
Here, the resistor, .
So, . Hence the voltage in the output will be

Thus, we can conclude from the above equation that the current I_L is related to the voltage, V_IN and the resistor, R.